Physical Chemistry of Intrinsic Reaction Rates

2 Physical Chemistry of Intrinsic Reaction Rates 2.2.1 Introdnction 

While the relationship between the electronic properties and the reaction enthalpy is important in understanding energetics, the more important thermodynamic feature to focus on is the free energy. Indeed, in Chapter 4 the maximum for the rate of a zeolite-catalyzed reaction is not found for the zeolite with the smallest pore size (maximum adsorption enthalpy) but for medium-sized micropores where adsorbates have a higher entropy, and as a consequence, their concentration is a maximum. The gain in entropy often balances the loss in adsorption enthalpy. 

In the subsections that follow we will focus on the factors that maximize the rate constant for elementary surface reaction steps. Again we will stress the need explicitly to include entropic contributions. According to transition-state reaction rate theoryl l, the rate of the elementary conversion step is defined as 

2 The Transition-State Theory Definition of the Reaction Rate Constant Loose and Tight Transition States 

In order to appreciate the use of transition-state rate expressions, it is important to be reminded of the different time scales of the processes that imderpin the chemistry we wish to describe. The electronic processes that define the potential-energy surface on which atoms move have characteristic times that are of the order of femtoseconds, 10 sec, whereas the vibrational motion of the atoms is on the order of picoseconds, 10 sec. The overall time scale for bond activation and formation processes that control catalysis vary between 10 and 10 sec. This implies that on the time scale of the elementary reaction in a catalytic process, many vibrational motions occur. If energy transfer is efficient, then the assumption that all vibrational modes except the reaction coordinate of the chemical reaction are equilibrated is satisfied. Kramersl l defined this condition as Eb 5kT. Under this condition the transition state reaction-rate expression applies  

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